For moderate temperatures e.g. 40˚s and warming, a vest is better than a jacket.

Most of the warmth, less flappy, & mine packs to about the size of an inner tube. The right rain jacket can also be made to fit in a jersey pocket.

For long rides, and variable conditions, it's also important to pick a jersey with bigger pockets-

some of the newer ones have stylishly little ones for a gel, a co2, and a credit card only.